FULLERENE DERIVATIVES IN POLY(METHYL METHACRYLATE) - AN EPR AND ZERO-FIELD ODMR STUDY OF THEIR PHOTOEXCITED TRIPLET-STATES

Samples of two fullerene derivatives, namely, C60C2H4N(CH3) and C60C2H 4N(Ph(2)C(6)H(4)-4-OCH3) in poly(methyl methacrylate) (PMMA), were pre pared either by solvent evaporation from CHCl3 solutions or by thermal polymerization of methyl methacrylate monomers containing the fullero pyrrolidines. In the first case PMMA behaves as a neutral polymeric ma trix, while in the second the material swells by absorption of the sol vent and its grass transition temperature is shifted up by 8 degrees C with respect to undoped material. Both pieces of evidence are typical of cross-linking between the polymer chains that involves fulleropyrr olidines. EPR and ODMR spectroscopies are applied to the study of the cross-linked species. It is found that its excited triplet state is ch aracterized by unusually large electron dipolar splitting (\D\ = 248 x 10(-4) cm(-1)). Calculations of D have been carried out using a model that considers C-3(60) and the fullerene derivatives as a collection of fully localized double bonds, The experimental negative sign of D h as been reproduced for C-3(60), and a positive sign of D is expected f or cross-linked fulleropyrrolidine affected by cross-links in the equa torial region. Such a sign reversal can explain the different spin pol arization patterns exhibited by their EPR spectra.